1. Field of the Invention
The present invention relates to an ultrasonic flowmeter for measuring the flow volume of a fluid by using an ultrasonic wave and a method of measuring the flow volume of a fluid. The present invention also relates to a gas meter.
2. Description of the Related Art
Ultrasonic flowmeters have advantages such as simplicity in structure, a smaller number of mechanical moving portions, a wide flow volume measurable range, no pressure loss due to use of the flowmeter, etc. The measuring accuracy of ultrasonic flowmeters can also be improved utilizing the recent advanced electronics. Thus, ultrasonic flowmeters are studied in various fields such as gas meter application, where the flow volume of a gas or a liquid must be measured.
The structure of a conventional ultrasonic flowmeter and the principle of measurement using the ultrasonic flowmeter will be described. FIG. 12 is a block diagram of an example of a conventional ultrasonic flowmeter. The conventional ultrasonic flowmeter is, for example, one disclosed in Japan Electric Measuring Instruments Manufacturer's Association Standard, JEMIS 5032 “Method of Measuring Flow volume by Ultrasonic Wave”, Japan Electric Measuring Instruments Manufacturer's Association (Public Corporation), 1987.
As shown in FIG. 12, ultrasonic transducers 1 and 2 are placed on opposite sides of a flow passage 14 through which a fluid flows. The ultrasonic transducers 1 and 2 respectively functions as a transmitter and a receiver. That is, if the ultrasonic transducer 1 is used as a transmitter, ultrasonic transducer 2 is used as a receiver. If the ultrasonic transducer 2 is used as a transmitter, ultrasonic transducer 1 is used as a receiver. As shown in FIG. 12, the ultrasonic propagation path formed between the ultrasonic transducers 1 and 2 is inclined by an angle θ from the direction of flow of the fluid.
In the case of propagation of an ultrasonic wave from the ultrasonic transducer 1 to the ultrasonic transducer 2, the ultrasonic wave travels in a forward direction relative to the flow of the fluid and the velocity of the ultrasonic wave is therefore increased. In the case of propagation of an ultrasonic wave from the ultrasonic transducer 2 to the ultrasonic transducer 1, the ultrasonic wave travels in a reverse direction with respect to the flow of the fluid and the velocity of the ultrasonic wave is therefore reduced. Therefore, the velocity of the fluid can be obtained from the difference between the time period during which the ultrasonic wave travels from the ultrasonic transducer 1 to the ultrasonic transducer 2 and the time period during which the ultrasonic wave travels from the ultrasonic transducer 2 to the ultrasonic transducer 1. Also, the flow volume can be obtained from the product of the sectional area of the fluid passage 14 and the flow velocity.
An example of a measurement method based on a sing-around method will be described as a method of obtaining the flow volume of a fluid on the basis of the above-described principle will be described.
As shown in FIG. 12, the ultrasonic flowmeter has a transmitting section 3 and a receiving section 6. The ultrasonic transducer 1 is selectively connected to one of the transmitting section 3 and the receiving section 6 by the switching section 10. At this time, the ultrasonic transducer 2 is connected to the other of the transmitting section 3 and the receiving section 6 to which the ultrasonic transducer 1 is not connected.
When the transmitting section 3 and the ultrasonic transducer 1 are connected, the transmitting section 3 drives the ultrasonic transducer 1 and the ultrasonic wave generated by the ultrasonic transducer 1 reaches the ultrasonic transducer 2 by traveling across the flow of the fluid. The ultrasonic wave received by the ultrasonic transducer 2 is converted into an electric signal. This received signal is amplified by a receiving section 6. A zero-cross detection section 7 detects a zero-cross point immediately after a time at which a predetermined level is reached by the received signal, and generates a zero-cross detection signal. The zero-cross point is a point at which the amplitude of the received signal changes from plus to minus or from minus to plus. This zero-cross point is assumed to be the time at which the ultrasonic wave reaches the ultrasonic transducer 2. On the basis of the zero-cross detection signal, a trigger signal is generated with a delay of a predetermined time period to be input the trigger signal to the transmitting section 3. The time period from the generation of the zero-cross detection signal to the generation of the trigger signal will be referred to as a delay time.
The transmitting section 3 drives the ultrasonic transducer 1 on the basis of the trigger signal to generate the next ultrasonic wave. This cycle in which a loop of transmission-reception-amplification and delay-transmission is repeated will be referred to as sing-around, and the number of times the loop is repeated will be referred to as the number of sing-arounds.
A time measurement section 9 measures the time taken to perform the loop a predetermined number of times, and sends the result of measurement to a flow volume computation section 11. The switching section 10 is then switched to use the ultrasonic transducer 2 as a transmitter and the ultrasonic transducer 1 as a receiver. Measurement is thereafter performed in the same manner as that described above.
A value obtained as the product of the delay time and the number of sing-arounds is subtracted from the time period measured by the above-described method, and the result of this subtraction is divided by the number of sing-arounds to obtain an ultrasonic wave propagation time. Let the propagation time when the ultrasonic transducer 1 is in the transmitting position be t1, and let the propagation time when the ultrasonic transducer 2 is in the transmitting position be t2.
Also, let the distance between the ultrasonic transducer 1 and the ultrasonic transducer 2 L, and let the flow velocity of the fluid and the sound velocity of the ultrasonic wave be V and C, respectively, as shown in FIG. 12.
Then t1 and t2 are expressed by the following equations:                     [                  Equation          ⁢                                           ⁢          1                ]                                                                       t1          =                      L                          C              +                              V                ⁢                                                                   ⁢                cos                ⁢                                                                   ⁢                θ                                                    ⁢                                  ⁢                  t2          =                      L                          C              -                              V                ⁢                                                                   ⁢                cos                ⁢                                                                   ⁢                θ                                                                        (        1        )            
From these equations, the flow velocity V is expressed by the following equation:                     [                  Equation          ⁢                                           ⁢          2                ]                                                             V        =                              L                          2              ⁢                                                           ⁢              cos              ⁢                                                           ⁢              θ                                ⁢                      (                                          1                t1                            -                              1                t2                                      )                                              (        2        )            
After the calculation of the flow rate V, the flow volume Q is obtained from the product of the sectional area of the flow passage 14 and the flow velocity V.
In the above-described ultrasonic flowmeter, zero-cross detection is used to measure the times t1 and t2 that ultrasonic waves propagate between the ultrasonic transducer 1 and the ultrasonic transducer 2. Therefore there must be coincidence between the waveform obtained when the ultrasonic wave transmitted from the ultrasonic transducer 1 is received by the ultrasonic transducer 2, and the waveform obtained when the ultrasonic wave transmitted from the ultrasonic transducer 2 is received by the ultrasonic transducer 1.
In actuality, however, the waveforms do not coincide perfectly with each other because of the characteristic difference between the ultrasonic transducer 1 and the ultrasonic transducer 2. FIG. 13 shows a waveform 18 obtained when the ultrasonic wave transmitted from the ultrasonic transducer 1 is received by the ultrasonic transducer 2 and a waveform 19 obtained when the ultrasonic wave transmitted from the ultrasonic transducer 2 is received by the ultrasonic transducer 1 in a case where the flow velocity of the fluid is zero and the ultrasonic transducer 1 and the ultrasonic transducer 2 differ in characteristics from each other. The zero-cross point in the waveform 18 immediately after a level a has been exceeded by the received signal is indicated as a point 18a, and the corresponding zero-cross point in the waveform 19 is indicated as a point 19a. These points do not coincide with each other. That is, an erroneous flow volume is indicated even when the gas or liquid to be measured is not flowing.
The ultrasonic transducer 1 is constituted by a piezoelectric element usually having a temperature dependent characteristic. FIG. 14 shows a temperature dependence of the difference Δt between the above-mentioned propagation times t1 and t2 when the flow velocity of the fluid is zero. In some cases, At changes generally in proportion to the temperature, as indicated by a curve 20a in FIG. 14, or increases or decreases abruptly with the increase in temperature, as indicated by a curve 20b or 20c. This is because the ultrasonic transducer 1 and the ultrasonic transducer 2 have different temperature dependent characteristics and hence the characteristic varies depending on the combination of the ultrasonic transducers.
Therefore, in the case of measurement with a gas meter incorporating the conventional ultrasonic flowmeter, an error due to the characteristic difference between the two ultrasonic transducers can cause a false indication that the gas is used in the case where a gas is not actually used. Also, the temperature dependence of the characteristic difference causes difference in the amounts of gas use measurement for a single gas appliance between different times, for example, a morning time when the atmospheric temperature is lowest in one day, and a time of the daytime when the atmospheric temperature rises.
Also, if the gas meter has a gas leak detection function, there can be not only an error in measurement but also a lowered reliability of the gas leak detection function.